Three types of electron conformal therapy (“ECT”) either require or could benefit from intensity modulated (IM) electron beams: segmented-field ECT, bolus ECT, and modulated electron radiation therapy (“MERT”); however, no reliable method for IM of ECT is available.
Intensity modulation of an X-ray beam has been accomplished using multileaf collimators (“MLCs”), which are collimators with an assemblage of “leafs” which selectively block sections of an X-ray beam. Attempts that have been made to deliver electron intensity modulation using photon MLCs have proven unsuccessful. When X-ray MLCs are used with electron beams, scattering in the large air gap between the MLCs and the PTV nullifies any useful modulation of the electron beam. Thus, photon MLCs have been ineffective in IM for ECT.
Electron multileaf collimators (“eMLC”) have been made and are commercially available, but because these devices are heavy, cumbersome, not permanently attached to the electron accelerator, and expensive, few if any eMLCs are used. Further, eMLCs must be placed very close to the patient, for example within 10-15 cm, which makes it even more difficult to use an eMLC. In comparison, photon MLCs typically are placed about 60-70 cm from the patient and are attached to the electron accelerator.
Proton beam IM, best achieved by modulating the beam as it is magnetically scanned across the PTV, is possible due to proton multiple Coulomb scattering (MCS) in air being of such small magnitude (few mm). Such is not the case for electrons. Even replacing the intervening air with a gas such as helium, which causes relatively low MCS, is insufficient due to MCS from the accelerator end window and ion chamber. Furthermore, scanning electron beams have remained off the market since the 1990s as a result of deadly accidents that occurred with scanned electron beam machines. These approaches are impractical for routine treating of patients, and thus to date, these approaches remain commercially unavailable.
Although not actually IM, when EBT is used to treat an eye retina, a single island is placed over the patient's eye. Such an island blocks the electron beam from directly impinging on the underlying lens, which is at a shallow depth (0.3 cm), thereby reducing the possibility of blindness or cataracts formation. However, electron MCS allows the underlying retina, which is at a deeper depth (3.0 cm), to receive about 70% of the prescribed dose.
Also not actually IM, on some occasions, EBT utilizes two or more electron beams having different energies (also known as Segmented Field ECT). However, due to differing beam energies or beam misalignment, unwanted hot spots and/or cold spots can result in the treatment volume. To reduce these unwanted results, saw-toothed collimator edges have been used to match the electron penumbra (e.g. 80%-20% dose edge) for abutting beams of differing energies.
The invention disclosed herein is a novel method for passive electron intensity modulation that comprises inserting a plurality of Island Blocks (sometimes referred to as “Islands”) and/or Island Apertures (sometimes referred to as “Apertures”) in the path of an electron beam. This method provides control of electron beam intensity modulation within the treatment field and in the penumbra. The pattern of Island Blocks and/or Island Apertures may be optimized for any particular EBT treatment. This method is low-cost and can be easily incorporated into most existing clinical settings.